The present invention relates to a printer having a thermal head, and more particularly to a serial thermal transfer printer or serial thermal printer which has a fast moving thermal head during printing.
In case of a thermal transfer printer or thermal printer, printing must be carried out so as to achieve a high speed in a state where a thermal head is moving with respect to an ink ribbon or heat sensitive paper. Particularly, in a serial thermal transfer printer or thermal printer, the thermal head is moving at a substantially constant speed during printing because of a high moving speed of the thermal head.
Therefore, as shown in FIG. 8, the printed dots of a letter m are elongated in the direction of movement of the thermal head. In FIG. 8, H1 and H2 indicate heat element widths; D1 and D2 indicate printed dot widths; g.sub.o indicates a gap between adjacent dots; and 2W.sub.o indicates successive 2 dots width.
The above laterally elongated form of the printed dots causes degradation of printing quality. Disadvantages are inherent in printing Chinese characters because they are not symmetrical and horizontal, part having white letters within black background, and vertical lines, such as letters m, w, M or W etc., tend to buckle. Thus the laterally elongated form of the printed dots degrades the printing quality and limiting the speed of the serial thermal transfer printer or thermal printer.
To improve the form of the printed dots, as shown in Japanese Utility Model Laid Open Publication No. 51-73043, there has been proposed a method such that each heat element on a thermal head is so designed that the dimension in the direction of movement of the thermal head is less than the dimension perpendicular to the direction of movement so that the printed dots are more nearly square.
However, this method has following drawbacks and has not been realized:
(1) applied power per unit area of the heat element is increased; hence pulse-resistant service life of the heat element is reduced;
(2) in a conventional wiring configuration where electrodes for energizing heat elements are led out laterally or in the direction of movement of a thermal head, the resistance value for each heat element is reduced so that the energizing current and the load of the driving element are both increased and the voltage drop due to common impedance is also increased, thereby adversely affecting printing quality; and
(3) when a film thickness of the heat element is reduced, as a countermeasure against the term (2), to increase the resistance value thereof, the pulse-resistant service life is reduced and when attempting to raise the specific resistance of the material, the degree of freedom of selection of materials is greatly restricted.